Conventionally known printing apparatuses print characters, images and the like on a sheet material (printing medium), e.g., paper, plastic film or the like. Image forming processes adopted by these printing apparatuses include: a wire dot method, a thermal transfer method, an inkjet method, a laser beam method, and so forth.
A printing apparatus of this type includes a serial printing type which performs printing by scanning a carriage incorporating a printhead, a line printing type which performs printing in units of lines using a line-head, and a page printing type which performs printing in units of pages.
Among these printers, the serial-type inkjet printer forms an image on a printing medium by mounting an inkjet printhead (hereinafter referred to as a printhead) on a carriage, which is reciprocally moved in the longitudinal direction of a platen using a carriage motor as a driving source, and driving a plurality of printing elements provided in the printhead in synchronization with scanning of the carriage, thereby discharging ink from ink discharge nozzles (hereinafter referred to as nozzles) which correspond to the printing elements.
Along with recent demands for high-quality printing, high-density printheads become common. For instance, an inkjet printing apparatus comprising a printhead which has nozzles for 600 dots per inch (600 dpi) is widely used. Furthermore, printing media that can be us d in printing are expanded from inkjet-exclusive paper, e.g., coated paper, to plain paper. As such high-density printheads are prevailing, it has become a normal procedure to receive bitmap image data and print a graphic image, or to receive image data sensed by a digital camera and print a photograph image.
Furthermore, in order to meet the recent demands for high-speed printing, printheads respectively discharging black (K), cyan (C), magenta (M) and yellow (Y) ink tend to have a large number of nozzles. Moreover, in place of multi-pass printing where printing is performed in units of bandwidth corresponding to the length of the nozzle column of a printhead, one-pass one-way printing where image printing is completed by a single scan of a carriage as shown in FIG. 8, or one-pass two-way printing shown in FIG. 9 are becoming common (refer to, e.g., Japanese Patent Application Laid-Open No. 9-226185).
FIGS. 8 and 9 show a printhead unit comprising four heads, each having 256 nozzles, for respectively discharging K, C, M and Y ink.
Furthermore, as printing speed increases, the carriage scanning speed becomes faster. As a result, acceleration and deceleration levels of the carriage tend to increase. Moreover, to meet demands for reduced noise of the printing apparatus, a DC motor is generally employed as a carriage motor.
However, because of the recent trend to downsize the printing apparatus, it is impossible to secure a long distance for carriage acceleration or deceleration. In a case of a printer whose size is constrained like a mobile printer, the acceleration or deceleration distance must be further reduced. In view of this, so-called acceleration/deceleration printing is becoming common. In the acceleration/deceleration printing, a printing operation is performed during acceleration/deceleration of the carriage, i.e., before the carriage reaches its constant speed, so that printing can be performed even in the left and right edges (several millimeters) of a printing medium having a maximum size printable by the printing apparatus. Particularly in a rimless printing mode where an image is printed on all areas of a printing medium, the acceleration/deceleration printing is essential.
However, in the above-described conventional example, since a printing operation is performed during carriage acceleration, the printhead driving current and the carriage-motor driving current reach their peaks at the same time. Depending on the printing density, it is often the case that the driving current reaches the maximum current of the power source that drives the printing apparatus. Particularly, as shown in FIG. 10, a DC motor has a characteristic of requiring a low driving current at the time of high-speed rotation, that is, constant-speed driving, but requires a high driving current at the time of low-speed rotation imposing a large load on the motor, that is, at the time of motor acceleration. Since the time during which the maximum current is generated varies from tens of milliseconds to hundreds of milliseconds depending on the printing conditions, this variation cannot be dealt by the electrolytic capacitor.
In order to supply such large amount of current, the power source capacity must be increased. This causes an increased cost of the power source, resulting in raising the cost of the printing apparatus.
If a current supplied to the DC motor is reduced in order not to increase the power source capacity, the carriage scanning speed declines, resulting in a reduced throughput.
Furthermore, in order to accelerate the carriage to a predetermined speed within a short acceleration/deceleration distance, it is necessary to enlarge the size of the carriage motor so as to improve its acceleration performance. However, this raises a problem of an increased size of the printing apparatus.